Gmail "550-5.7.1 ... likely unsolicited mail": why it happens and how to fix it

What it means

Gmail received your email, looked at it, and decided it was probably spam. Instead of putting it in the Spam folder, it refused it completely. The bounce message usually looks like this:

550-5.7.1 [203.0.113.10] Our system has detected that this message is
550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail,
550-5.7.1 this message has been blocked. For more information, go to
550 5.7.1 https://support.google.com/mail/?p=UnsolicitedMessageError

The code 550 means a permanent refusal: Gmail will not retry. The IP in brackets is the server that delivered your email to Gmail.

Why it happens

Gmail does not publish its exact rules, but this error is almost always caused by one or more of these:

• Missing or broken authentication. SPF, DKIM or DMARC fails, so Gmail cannot be sure the email really comes from your domain.
• Poor reputation. Your sending IP or domain has a history of spam complaints, or it is new and has no history at all.
• Spam-like content. Shortened links, links to domains with a bad reputation, a lot of images with little text, or wording typical of scams.
• Blacklisted links or IP. A domain in your links, or your IP, is listed on a public blacklist.
• Sudden volume. A big send from a server that usually sends very little.

Often it is a combination: a weak reputation plus one suspicious element is enough.

How to check

• Send a test email: on the Delivmail home page (/), copy your unique test address and send it a normal email from the same server, tool or mailbox that bounced. Open the report when it is ready.
• Read the report. Look at SPF, DKIM and DMARC first, then the blacklist results for your IP and for the domains found in your links, and the SpamAssassin score.
• Check your IP and domain on /blacklists.
• Look at Google Postmaster Tools (postmaster.google.com) if you send a lot to Gmail. It shows your domain reputation and spam rate as seen by Gmail.

How to fix it

1. Fix authentication first. Publish one valid SPF record that includes your sending server, turn on DKIM signing in your email tool, and publish a DMARC record (start with p=none). The From domain must match the domain checked by SPF or DKIM.
2. Clean your links. Remove URL shorteners and links to any domain that appears on a blacklist. Link to your own domain where you can.
3. Improve the content. Write a real text part, keep a sensible balance between text and images, avoid all-caps and "too good to be true" wording, and fix anything the SpamAssassin section flags.
4. Only send to people who asked. Remove old and unengaged addresses, and add a clear unsubscribe link to marketing emails.
5. If you use a shared host, the problem may come from other customers on the same IP. Ask your provider, or switch to a reputable email service.
6. Warm up new IPs and domains. Start with small volumes to your most engaged contacts and increase step by step.

Check that it is fixed

Send a new Delivmail test and confirm that SPF, DKIM and DMARC pass and that nothing is blacklisted. Then send one normal email to the Gmail address that bounced. If reputation was the cause, it can take several days of clean sending before Gmail accepts your emails again. Delivery is never guaranteed, but a clean report removes the most common reasons for this block.